HVAC system with electronically controlled expansion valve

ABSTRACT

A two temperature electronic expansion valve control for variable speed compressors that utilizes a correlation between airflow percentage and heat exchanger pressure drop to control the operation of an expansion valve. An indoor airflow percentage request may be communicated from an outdoor controller to an air handler controller. Using a correlation between airflow percentage and pressure drop across the heat exchanger, the airflow percentage may be used in predicting an outlet pressure of refrigerant exhausted from the heat exchanger. The predicted pressure drop may be used in determining a saturated temperature for the exhausted refrigerant. The determined saturated temperature may be compared to a sensed temperature of the refrigerant at the outlet of the heat exchanger to determine a superheat value, which is compared to a reference superheat value in determining the degree to open or close the expansion valve.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/875,031, filed Sep. 7, 2013, which isincorporated herein by reference in its entirety.

BACKGROUND

Embodiments of the present invention generally relate to refrigerationsystems. More particularly, embodiments of the present invention relateto controlling the operation of an electronically controlled expansionvalve for HVAC systems.

HVAC systems with air handlers, including, for example, air-conditionersand heat pumps, among other HVAC systems, often have an electronicexpansion valve (EEV) to control refrigerant flow. The degree to whichthe EEV is opened or closed may control the amount of refrigerant thatflows through the HVAC system, and thereby control the superheat leavinga heat exchanger, such as, for example, an evaporator. Moreover, the EEVmay assist in controlling the flow of refrigerant in the HVAC system sothat an appropriate amount of superheat leaves the evaporator. Superheatleaving the evaporator is typically defined as the difference betweenrefrigerant gas temperature leaving the evaporator and saturatedrefrigerant temperature leaving the evaporator.

Determination of the superheat leaving the evaporator often involves, atleast in part, a determination of a pressure drop in the refrigerantacross the evaporator. Further, traditionally, many HVAC systems are twospeed systems, and more specifically, refrigeration flow is eithertypically at a high speed or a low speed. Thus, for such systems, afirst, high speed pressure drop value for refrigerant across theevaporator is typically anticipated when the system is operating in thehigh speed mode, and a different second, low speed pressure value isanticipated when the system is operating in the low speed mode. Further,the first, high speed pressure drop value and the second, low speedpressure drop value are often fixed values in that the values are notadjusted to accommodate for changes in the HVAC system.

However, for variable flow systems, refrigerant flow typicallycontinuously varies. Therefore, using fixed values for anticipatedpressure drop of refrigerant across the evaporator may result ininaccurate superheat values. Yet, inaccurate superheat values may causeover or under compensation in the positioning of the EEV, therebyresulting in the EEV releasing too much, or, alternatively, not enough,refrigerant into the refrigerant flow. Thus, there is a need to developa means to continuously determine the pressure drop across a heatexchanger, such as, for example, the pressure drop of refrigerant acrossan evaporator, as the flow rate of refrigerant in the HVAC systemvaries.

BRIEF SUMMARY

An aspect of the present invention is an HVAC system having a heatexchanger and a fan, the fan being adapted to flow air toward the heatexchanger. Additionally, the fan is controlled in response to an airflowpercentage request. The HVAC system further includes an expansion valvethat is in fluid communication with the heat exchanger, the expansionvalve being configured to control a flow of a refrigerant. Additionally,the HVAC system includes a controller that is adapted to control theoperation of the expansion valve based at least in part on a correlationbetween a drop in a pressure in the heat exchanger and the airflowpercentage request.

Another aspect of the present invention is a method for controlling therelease of refrigerant from an electronically controlled expansionvalve. The method includes determining an airflow percentage for theoperation of a fan, the airflow percentage corresponding to a quantityor rate of flow of air to a heat exchanger. Further, a correlationbetween airflow percentage and pressure drop across the heat exchangeris used to determine an estimated pressure drop. Additionally, an outletpressure for refrigerant exhausted from the heat exchanger is determinedbased on an inlet pressure of refrigerant at an inlet of the heatexchanger and the estimated pressure drop. The method further includesdetermining, using at least the outlet pressure, a heat exchanger exitsaturation temperature and sensing, by an outlet temperature sensor, anoutlet temperature for refrigerant that is exhausted from the heatexchanger. A comparison of the sensed outlet temperature and the heatexchanger exit saturation temperature is used to derive an estimatedsuperheat value. Further, a flow of refrigerant released by theelectronically controlled expansion valve is adjusted based on anoutcome of the comparison between the derived estimated superheat valueand the reference superheat value.

Another aspect of the present invention is a method for controlling therelease of refrigerant from an electronically controlled expansionvalve. The method includes determining, by at least one controller, anairflow percentage for a flow of air by a fan to an evaporator. Using atleast the determined airflow percentage and a relationship betweenairflow percentage and refrigerant pressure drop across the evaporator,an estimated refrigerant pressure drop across the evaporator isdetermined. The method further includes determining a saturatedtemperature for refrigerant exhausted from the evaporator using at leastthe estimated refrigerant pressure drop across the evaporator andsensing, by a temperature sensor, an outlet temperature of refrigerantthat is exhausted from the evaporator. Additionally, the saturatedtemperature and the sensed outlet temperature of refrigerant arecompared to determine an estimated superheat value that is compared to areference superheat value. The method further includes adjusting therelease of refrigerant from the electronically controlled expansionvalve based on the outcome of the comparison between the estimatedsuperheat value and the reference superheat value.

Another aspect of the present invention is an HVAC system having anelectronically controlled expansion valve. Other embodiments includeapparatuses, systems, devices, hardware, methods, and combinations forHVAC systems. Further embodiments, forms, features, aspects, benefits,and advantages of the present application will become apparent from thedescription and figures provided herewith.

BRIEF DESCRIPTION OF THE DRAWINGS

The description herein makes reference to the accompanying figureswherein like reference numerals refer to like parts throughout theseveral views.

FIG. 1 illustrates a schematic diagram of some aspects of a non-limitingexample of an HVAC system in accordance with an embodiment of thepresent invention.

FIG. 2 illustrates an exemplary plot depicting a relationship betweenevaporator pressure drop and airflow percentage in accordance with anembodiment of the present invention.

FIG. 3 illustrates a flow chart for controlling the operation of anexpansion valve according to an illustrated embodiment of the presentinvention.

FIG. 4 illustrates a diagrammatic depiction of some aspects of theoperation of a refrigeration system in accordance with an embodiment ofthe present invention.

The foregoing summary, as well as the following detailed description ofcertain embodiments of the present invention, will be better understoodwhen read in conjunction with the appended drawings. For the purpose ofillustrating the invention, there is shown in the drawings, certainembodiments. It should be understood, however, that the presentinvention is not limited to the arrangements and instrumentalities shownin the attached drawings.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

For purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings, and specific language will be used to describe the same.It will nonetheless be understood that no limitation of the scope of theinvention is intended by the illustration and description of certainembodiments of the invention. In addition, any alterations and/ormodifications of the illustrated and/or described embodiment(s) arecontemplated as being within the scope of the present invention.Further, any other applications of the principles of the invention, asillustrated and/or described herein, as would normally occur to oneskilled in the art to which the invention pertains, are contemplated asbeing within the scope of the present invention.

FIG. 1 illustrates some aspects of a non-limiting example of an HVACsystem 10 in accordance with an embodiment of the present invention.According to certain embodiments, the HVAC system 10 includes an outdoorheat exchanger unit 12 and an indoor heat exchanger unit 14, such as,for example, an air handler. In other embodiments, the HVAC system 10may be housed in a single enclosure, or may take other forms that may besolely indoors or solely outdoors, or extend between indoors andoutdoors. Additionally, according to certain embodiments, the HVACsystem 10 is a refrigeration system of any type that employs anexpansion valve and an evaporator.

In the illustrated embodiment in which the HVAC system 10 is arefrigeration system, the outdoor heat exchanger unit 12 includes acompressor 16 and a condenser 18. According to certain embodiments, thecompressor 16 is a variable speed compressor. The indoor heat exchangerunit 14 may include an expansion valve 20, an evaporator 22, and a fanor blower 24. Additionally, the HVAC system 10 may include one or morecontrollers, such as, for example, a first controller 26, a secondcontroller 28, a third controller 30. In the illustrated embodiment, thefirst controller 26 is shown as being part of the outdoor heat exchangerunit 12, while the second and third controllers 28, 30 are part of theindoor heat exchanger unit 14. Further, the third controller 30 can becommunicatively coupled to the expansion valve 20 to form anelectronically-controlled expansion valve (EEV) 38.

Although the controllers 26, 28, 30 are illustrated in FIG. 1 asseparate components, according to other embodiments, one or more of thecontrollers 26, 28, 30 may be combined or may have their functionsdistributed across one or more other controllers, and may receive otherinputs and/or provide other outputs beyond those illustrated anddescribed herein. Additionally, the controllers 26, 28, 30 may bepositioned at a variety of locations, including with the outdoor heatexchanger unit 12 and/or the indoor heat exchanger unit 14. Thecontrollers 26, 28, 30 may be adapted to perform a variety of functionsrelating to the operation of the HVAC system 10. For example, accordingto certain embodiments, the one or more controllers 26, 28, 30 areadapted to sense room temperature, determine system demand, determinecommands for operation of various components of the HVAC system 10, andissue those determined commands, among other functions.

The HVAC system 10 may further include a thermostat 32 that is operablycoupled to the outdoor heat exchanger unit 12 and/or the indoor heatexchanger unit 14. For example, in the illustrated embodiment, thethermostat 32 is coupled to the first controller 26 and the secondcontroller 28 such that commands and information may be at leastcommunicated from, and/or between, the thermostat 32 and the first andsecond controllers 26, 28.

The HVAC system 10, and in particular the indoor heat exchanger unit 14,also includes an inlet temperature sensor 34 and an outlet temperaturesensor 36. As shown in FIG. 1, according to the illustrated embodiment,the inlet and outlet temperature sensors 34 and 36 may becommunicatively coupled to the third controller 30, which may be part ofthe indoor heat exchanger unit 14. Further, the inlet temperature sensor34 may be disposed adjacent to, or upstream of, the inlet of evaporator22. For example, according to certain embodiments, the inlet temperaturesensor 34 is coupled to a distributor tube at the inlet 25 to theevaporator 22 to measure saturated refrigerant temperature of therefrigerant that is being delivered to the evaporator 22. Thus,according to the illustrated embodiment, the inlet temperature sensor 34is positioned to sense the temperature of refrigerant at that is beingdelivered to, or is received by, the evaporator 22. Similarly, theoutlet temperature sensor 36 is positioned to sense the temperature ofrefrigerant that is being, or has been, exhausted from the evaporator22. For example, the outlet temperature sensor 36 may be coupled to arefrigerant tube that is leaving the evaporator 22, and thereby theoutlet temperature sensor 36 may measure a gas temperature of theexhausted refrigerant.

According to the illustrated embodiment in which the HVAC system 10 is arefrigeration system, a refrigerant is compressed by the compressor 16,rejects heat at the condenser 18, and expands in the expansion valve 20.Refrigerant released from the expansion valve 20 may flow to theevaporator 22, where the temperature of the refrigerant may be sensed bythe inlet temperature sensor 34. The refrigerant may then retain heat atthe evaporator 22 before being exhausted from the evaporator 22.Further, according to the illustrated embodiment, the amount of heatretained by the refrigerant in the evaporator 22 may be at leastpartially controlled by controlling the volumetric flow of air to orover the evaporator 22. Thus, control of the volumetric flow of airdelivered to the evaporator 22 may be controlled via controlling theoperation of the fan 24. Additionally, as previously discussed, thetemperature of the refrigerant that is, or is being, exhausted from theevaporator 22 is sensed by the outlet temperature sensor 36 at leastbefore the refrigerant is returned to the compressor 16.

According to the certain embodiments, the first controller 26 may be anoutdoor controller. Moreover, according to certain embodiments, thefirst controller 26 may be an airflow control board. Further, as shownin FIG. 1, the first controller 26 may be communicatively coupled to thecompressor 16 such that the first controller 26 may operably supplysignals for controlling the operation of the compressor 16.Additionally, the first controller 26 may also be communicativelycoupled to the second controller 28. According to such an embodiment,the first controller 26 is operative to supply an airflow request, suchas, for example, a request communicating airflow percentage that is tobe attained by operation of the fan 24, to the second controller 28.

According to certain embodiments, the airflow percentage may correspondto an operating parameter of the fan 24, or to a characteristic of theairflow being blown by the fan 22 to, and/or about, the evaporator 22.Further, according to certain embodiments, the airflow percentage may beproportional to the speed/capacity of the compressor 16. For example,according to certain embodiments, the airflow percentage may correspondto a volumetric flow rate for the airflow that is to flow to theevaporator 22 via operation of the fan 24. According to otherembodiments, the airflow percentage may correspond to a speed of the fan24, such as, for example, a duty cycle.

According to the illustrated embodiment, the second controller 28 may bean air handler controller. Further, the second controller 28 iscommunicatively coupled to the fan 24. As previously discussed, the fan24 is configured to provide an airflow to, and/or about at least aportion of, the evaporator 24. Moreover, according to the illustratedembodiment, the second controller 28 is operative to control the outputof fan 24 based on the airflow request received from first controller26. For example, according to certain embodiments, the second controller28 is configured to the control the speed at which the fan 24 operatesbased on information contained in the airflow percentage request. Bycontrolling the speed of the fan 24, the second controller 28 maycontrol the volumetric flow of air that the fan 24 blows to and/or aboutthe evaporator 22, and thereby control the temperature of the airflowthat is exhausted, or flows away, from the evaporator 22.

The second controller 28 is communicatively coupled to the thirdcontroller 30. As previously discussed, in the illustrated embodiment,the third controller 30 can be communicatively coupled to the expansionvalve 20 to form the electronically-controlled expansion valve (EEV) 38.According to such an embodiment, the third controller 30 may be used tocontrol the flow of refrigerant that is released from the expansionvalve 20. For example, the third controller 30 may be adapted to controlthe degree to which the expansion valve 20 is open or closed. Accordingto certain embodiments, the third controller 30 may be operablyconnected to an actuator of the expansion valve, such as, for example, amotor, that is adapted to alter the position of a valve component of theexpansion valve 38 that is used to provide varying degrees, if any, torelease of refrigerant from the expansion valve 20.

FIG. 2 illustrates an example of a correlation 40 between an evaporatorreference pressure drop and an airflow percent. Moreover, as airflowpercentage may be directly related to refrigerant flow rate, airflowpercentage may be used as a surrogate, with use of the correlation 40,to estimate an evaporator 22 pressure drop, and more specifically, adrop in refrigerant pressure, at a number of operating points along theevaporator 22. Thus, the airflow percentage may permit a determinationof pressure drop of refrigerant across the evaporator 22 as refrigerantflow in the system 10 varies. Moreover, the evaporator referencepressure drop may indicate an estimated or predicted pressure drop, andmore specifically, a refrigerant pressure drop, between a first pressureat or around the inlet 25 of the evaporator 22, and a second pressure oraround the outlet 27 of the evaporator 22.

According to certain embodiments, one or more correlation algorithms,look-up tables, models, maps, or other correlation sources may bederived empirically and/or by calculation, including, for example, byhand calculations, actual or virtual testing, and/or computer modeling.For example, according to certain embodiments, testing and/or modelingof the impact the airflow percentage has on refrigerant pressure dropacross the evaporator 22 at a variety of different operating conditionsmay be used to derive a correlation algorithm that corresponds to thecorrelation between airflow percentage and pressure drop across theevaporator 22. Alternatively, such testing and/or modeling may be usedto derive one or more look-up tables that provide the refrigerantpressure drop across the evaporator 22. The derived correlationalgorithm(s), table(s), maps, or other correlation 40 providingresources may be stored in one or more of the controllers 26, 28, 30,such as, for example in a memory of one or more of the controllers 26,28, 30.

The EEV 38 is configured to regulate the superheat of the refrigerant ata desired location in system 10, such as, for example, the superheat ofthe refrigerant that is exiting the evaporator 22. Therefore, accordingto certain embodiments, the EEV 38 may store, such as, for example, in amemory associated with, or otherwise in communication with, the thirdcontroller 30, a set-point or reference superheat value. The set-pointor reference superheat value may correspond to a superheat value orlevel that the system 10 may seek to attain/maintain. Moreover, theset-point or reference superheat value may provide a measuring point fordetermining whether the superheat being estimated or predicted as beingattained by the system 10 requires an adjusted in the operation of theexpansion valve 20, and more specifically, a change in the flow ofrefrigerant from the expansion valve 20. Further, according to theillustrated embodiment, the EEV 38 may also store, or otherwise haveaccess to, one or more properties of the refrigerant, such as, forexample, thermodynamic properties of the refrigerant.

FIGS. 3 and 4 illustrate an exemplary process 50 for controlling the EEV38 according to an embodiment of the present invention. While FIG. 3 isshown and described herein as containing certain steps, according toother embodiments, certain steps may be removed and/or added.Additionally, according to certain embodiments, the order in which stepsoccur may be different for different embodiments.

At step 52, the first controller 26 may receive one or more inputs ordemands. The inputs or demands may be associated with a number ofconditions or settings. For example, referencing FIGS. 3 and 4, suchdemand may be based on a demand signal communicated to the firstcontroller 26 from the thermostat 32. Such a demand from the thermostat32 may be associated with maintaining the associated building orstructure at a reference temperature, and/or a change in the referencetemperature. Additionally, as shown in FIG. 4, the first controller 26may receive a demand associated with an outdoor temperature. At step 54,based on at least the received demands or inputs, the first controller26 may determine an airflow requirement, such as, for example an airflowpercentage that is to be delivered by fan 24. Further, at step 54, thefirst controller 26 may also determine a flow rate for air that is to bedelivered to, or about, the condenser 18. At step 56, based on at leastsimilar demands that were used to determine the airflow percentage, thefirst controller 26 may also determine an operating speed for thecompressor 16.

As shown in FIG. 3, and further illustrated in FIG. 4, at step 58, thefirst controller 26 may communicate the airflow percentage request to atleast the second controller 28 and/or to the thermostat 32. At step 60,the second controller 28 directs the operation of the fan 24 based onthe airflow percentage request. For example, the second controller 28may transmit a command, signal, information, data, voltage, or the liketo the fan 24 that is used to control the operation of the fan 24, suchas, for example, the speed at which the fan 24 is operating.

According to the illustrated embodiment, at step 62, the airflowpercentage and/or the airflow percentage request is communicated to thethird controller 30. According to the illustrated embodiment, theairflow percentage and/or the airflow percentage request is communicatedto the third controller 30 by the second controller 28. However,according to other embodiments, the airflow percentage and/or theairflow percentage request is communicated to the third controller 30 bythe first controller 26.

At step 64, the inlet pressure (P_(i)) of the evaporator 22, and morespecifically, the pressure of refrigerant at, or adjacent to, the inlet25 of the evaporator 22 may be ascertained by one or more of thecontrollers 26, 28, 30, such as, for example, by the third controller30. For example, because the refrigerant supplied to the evaporator 22is a saturated mixture, and based on known thermodynamic properties ofthe refrigerant, the inlet pressure (P_(i)) of the refrigerant at oraround the inlet 25 of the evaporator 22 may be ascertained. At step 66,using the inlet temperature sensor 34, the temperature (T_(i)) of therefrigerant at or adjacent to the inlet 25 of the evaporator 22 may besensed or otherwise determined. According to certain embodiments, ratherthan determining the inlet pressure (P_(i)) of the refrigerant at theinlet 25 of the evaporator 22 as discussed above with respect to step 64based on the refrigerant being a saturated mixture, the temperaturesensed at step 66 by the inlet temperature sensor 34, as well as thethermodynamic properties of the refrigerant, may be used by one or moreof the controllers 26, 28, 30 to determine the inlet pressure (P_(i)) ofthe refrigerant.

At step 68, with the inlet pressure of the refrigerant determined, thecorrelation 40 between airflow percentage and pressure drop across theevaporator 22 may be employed. Moreover, the derived correlation 40allows for the use of the previously determined airflow percentage todetermine the predicted or estimated pressure change or drop (ΔP) of therefrigerant across the evaporator 22, as illustrated in FIG. 2. Forexample, as discussed above, according to certain embodiments, a memoryof one or more of the controllers 26, 28, 30, such as, for example, thethird controller 30, may contain one or more correlation algorithms ortables that allow for determination of the pressure drop (ΔP) using theairflow percentage. At step 70, an outlet pressure (P_(o)) of theevaporator 22, and more specifically, the pressure of refrigerant at, oradjacent to, the outlet 27 of the evaporator 22 may be derived, such as,for example, by the third controller 30, by subtracting the estimatedpressure drop (ΔP), as determined at step 68, from the inlet pressure(P_(i)).

At step 72, using the determined outlet pressure (P_(o)) and thethermodynamic properties of the refrigerant, one or more of thecontrollers 26, 28, 30, such as, for example, the third controller 30,may determine an evaporator exit saturation temperature (T_(sat)) forthe refrigerant at the outlet 27 of the evaporator 22. Additionally, atstep 74, an outlet temperature (T_(o)) for refrigerant that is exitingthe evaporator 22 may be sensed by the outlet temperature sensor 36. Atstep 76, a controller 26, 28, 30, such as, for example, the thirdcontroller 30, may evaluate or compare the calculated evaporator exitsaturation temperature (T_(sat)) with the sensed or measured outlettemperature (T_(o)) to derive an estimated superheat value (S_(est)). Atstep 78, a controller 26, 28, 30, such as the third controller 30,compares the derived estimated superheat value (S_(est)) to a referencesuperheat value (S_(ref)). According to certain embodiments, thereference superheat value (S_(ref)) may be stored in the memory of oneor more of the controllers 26, 28, 30, such as, for example, a memoryof, or accessible to, the third controller 30. Differences between theestimated superheat value (S_(est)) and the reference superheat value(S_(ref)) may provide an indication of whether the amount or rate ofrefrigerant that is flowing into at least the evaporator 22 is to beincreased or decreased. Moreover, the estimated superheat value(S_(est)) may provide feedback for comparison with the set-pointsuperheat value (S_(ref)), to determine the degree, if any, theexpansion valve 20 should be opened and closed, and thereby control theflow of refrigerant that is being released from the expansion valve 20.

At step 80, based on the results of the comparison of between theestimated superheat value (S_(est)) and the reference superheat value(S_(ref)), the expansion valve 20 may, or may not, be adjusted so as toalter the amount of refrigerant that is flowing through the expansionvalve 20. Additionally, the process 50 may be continuous so that thedegree that the expansion valve 20 is opened/closed can vary as flowconditions of the refrigerant changes. Such consideration of continuouschanges in refrigerant flow may allow for a more accurate pressurecalculation for refrigerant that is leaving the evaporator, and thusresult in better superheat control.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment(s), but on the contrary, is intended to covervarious modifications and equivalent arrangements included within thespirit and scope of the appended claims, which scope is to be accordedthe broadest interpretation so as to encompass all such modificationsand equivalent structures as permitted under the law. Furthermore itshould be understood that while the use of the word preferable,preferably, or preferred in the description above indicates that featureso described may be more desirable, it nonetheless may not be necessaryand any embodiment lacking the same may be contemplated as within thescope of the invention, that scope being defined by the claims thatfollow. In reading the claims it is intended that when words such as“a,” “an,” “at least one” and “at least a portion” are used, there is nointention to limit the claim to only one item unless specifically statedto the contrary in the claim. Further, when the language “at least aportion” and/or “a portion” is used the item may include a portionand/or the entire item unless specifically stated to the contrary.

The invention claimed is:
 1. An HVAC system comprising: a heatexchanger; a fan adapted to flow air toward the heat exchanger, the fancontrolled in response to an airflow percentage request; an expansionvalve in fluid communication with the heat exchanger, the expansionvalve configured to control a flow of a refrigerant; and a controllerconfigured to control the operation of the expansion valve, thecontroller having a memory that includes a correlation between a drop ina pressure in the heat exchanger and the airflow percentage request, thecontroller configured to form a control signal to the expansion valvebased on the correlation stored in the memory of the controller, and thecontroller further configured to determine an outlet pressure of therefrigerant exhausted from the heat exchanger based in part on thecorrelation between the drop in the pressure in the heat exchanger andthe airflow percentage request where airflow percentage request is usedas input to the correlation and drop in the pressure is an output fromthe correlation, such that the outlet pressure is determined based on aninlet pressure of refrigerant at an inlet of the heat exchanger and isdetermined on the basis of the drop in pressure which is output from thecorrelation.
 2. The HVAC system of claim 1, wherein the airflowpercentage request corresponds to a quantity or rate of air that the fanis to flow to the heat exchanger.
 3. The HVAC system of claim 2, whereinthe drop in the pressure is a drop of a refrigerant pressure, thecontroller being further adapted to determine an evaporator exitsaturation temperature based on at least the outlet pressure.
 4. TheHVAC system of claim 3, further including an outlet temperature sensoradapted to sense an outlet temperature of refrigerant exhausted from theheat exchanger, and wherein the controller is further adapted toevaluate the outlet temperature and the evaporator exit saturationtemperature to determine an estimated superheat value.
 5. The HVACsystem of claim 4, wherein the controller is further adapted to issue acommand to adjust the flow of refrigerant based on a comparison of theestimated superheat value to a reference superheat value.
 6. The HVACsystem of claim 4, wherein the heat exchanger is an evaporator, andfurther including an inlet temperature senor adapted to sense an inlettemperature of refrigerant being received by the heat exchanger, andwherein the controller is adapted to determine an inlet pressure usingat least the sensed inlet temperature, the controller further adapted todetermine the outlet temperature using at least the sensed inlettemperature and the correlation between drop of the pressure in the heatexchanger and the airflow percentage request.
 7. The HVAC system ofclaim 6, wherein the heat exchanger is part of an indoor exchanger unit,and further including an outdoor heat exchanger unit having a condenserand a compressor.
 8. The HVAC system of claim 7, wherein the controlleris further adapted to determine the airflow percentage request and anoperating speed of the compressor based on one or more demands on theHVAC system.
 9. A method for controlling the release of refrigerant froman electronically controlled expansion valve, the method comprising:determining an airflow percentage for the operation of a fan, theairflow percentage corresponding to a quantity or rate of flow of air toa heat exchanger; determining an estimated pressure drop across the heatexchanger using a direct correlation which is stored in a memory of acontroller where the direct correlation is between airflow percentageand estimated pressure drop across the heat exchanger and where airflowpercentage is used as input to the direct correlation and estimatedpressure drop is an output from the direct correlation; determining anoutlet pressure for refrigerant exhausted from the heat exchanger basedon an inlet pressure of refrigerant at an inlet of the heat exchangerand the estimated pressure drop; determining, using at least the outletpressure, a heat exchanger exit saturation temperature; sensing, by anoutlet temperature sensor, an outlet temperature for refrigerantexhausted from the heat exchanger; deriving, from a comparison of thesensed outlet temperature and the heat exchanger exit saturationtemperature, an estimated superheat value; and adjusting a flow ofrefrigerant released by the electronically controlled expansion valvebased on an outcome of the comparison between the derived estimatedsuperheat value and the reference superheat value.
 10. The method ofclaim 9, wherein the heat exchanger is an evaporator.
 11. The method ofclaim 10, wherein the step of determining the airflow percentageincludes receiving, by a controller, one or more system demands orinput, and further including the step of operating the fan to attain thedetermined airflow percentage.
 12. The method of claim 11, furtherincluding the step of sensing, by an inlet temperature sensor, an inlettemperature of refrigerant being received by the heat exchanger.
 13. Themethod of claim 12, further including the step of determining the inletpressure of refrigerant received by the heat exchanger using the sensedinlet temperature.
 14. The method of claim 13, further including thesteps of communicating the airflow percentage as an airflow percentagerequest from an outdoor controller to an air handler controller anddetermining, by the outdoor controller, an operating speed for acompressor, the compressor being in fluid communication with theelectronically controlled expansion valve.
 15. The method of claim 14,wherein the correlation between airflow percentage and estimatedpressure drop across the heat exchanger is a correlation algorithmstored in a memory of at least one of the at least one controller.
 16. Amethod for controlling the release of refrigerant from an electronicallycontrolled expansion valve, the method comprising: determining, by atleast one controller, an airflow percentage for a flow of air by a fanto an evaporator; determining an outlet pressure of refrigerantexhausted from the evaporator using (1) an inlet pressure of refrigerantto the evaporator, and (2) the determined airflow percentage and acomputational relationship stored in a memory of the at least onecontroller between airflow percentage and an estimated refrigerantpressure drop across the evaporator where airflow percentage is used asinput to the computational relationship and estimated refrigerantpressure drop is an output from the computational relationship;determining a saturated temperature for refrigerant exhausted from theevaporator using at least the estimated refrigerant pressure drop acrossthe evaporator; sensing, by a temperature sensor, an outlet temperatureof refrigerant exhausted from the evaporator; comparing the saturatedtemperature and the sensed outlet temperature of refrigerant todetermine an estimated superheat value; comparing the estimatedsuperheat value to a reference superheat value; and adjusting therelease of refrigerant from the electronically controlled expansionvalve based on the outcome of the comparison between the estimatedsuperheat value and the reference superheat value.
 17. The method ofclaim 16, wherein the step of determining the airflow percentageincludes, receiving, by the at least one controller, one or more systemdemands or inputs, and further including the step of operating the fanto attain the determined airflow percentage.
 18. The method of claim 17,wherein the relationship between airflow percentage and estimatedrefrigerant pressure drop across the evaporator is an algorithm storedin a memory of at least one of the at least one controller.
 19. Themethod of claim 17, wherein the airflow percentage is determined by anoutdoor controller, and further including the step determining, by theoutdoor controller, an operating speed for a compressor, the compressorbeing in fluid communication with the electronically controlledexpansion valve.
 20. The method of claim 19, further including the stepsof sensing, by an inlet temperature sensor, an inlet temperature ofrefrigerant being received by the evaporator, and determining an inletpressure for refrigerant being received by an evaporator using thesensed inlet temperature.